Smells, Gases Keep Order in Ant Colonies

When she first started looking at ant colonies in the Arizona desert 20 years ago, Deborah M. Gordon wasn't just interested in ants. She wanted to know if the ants could help answer some of the most difficult questions confronting scientists in a wide range of fields.

"I was interested in what is now called complex systems," says Gordon, now an associate professor of biology at Stanford University. "That's a system in which the units don't know globally what's going on, there's no central control, there's no hierarchy, there's nobody in charge, and yet somehow all those fairly simple units working together do something interesting."

The ants were supposed to serve as the vehicle for studying far more important issues, like how a brain works, but somewhere along the way the ants swept Gordon off her feet. She became so fascinated with these little critters that she has turned out one discovery after another, including a recent finding published in the May 1 issue of Nature.

Gordon and a fellow researcher, Michael J. Greene, used some highly sophisticated equipment and a little bit of trickery to figure out how ants know what they are supposed to do when nobody is in charge.

Odor Order

It all comes down to an extraordinary sense of smell, the researchers say. Ants that go out looking for food, called "patroller ants," emit a distinctive odor when they return to the nest, thus telling other ants, called "foragers," to go collect the bounty. It's not an order, Gordon says, but the foragers know what to do when ants return to the nest at the right pace, smelling like patrollers.

"That tells them the conditions are right," Gordon says, and the foragers go forth to do their thing without needing to be told to do so.

It's a little like how a brain functions, or how an embryo grows into a distinct organism.

In the human brain, Gordon says, "neurons don't know how to think," but billions of these crucial "units," working together, "manage to think and remember."

And how does an embryo, be it an unborn child or an ant, know what to do?

"No one says cell number 42, you be liver, cell number 76, you be bone, but somehow embryos develop into organisms with different kind of tissues," Gordon says.

Roles on the Hill

Studying these kinds of complex systems is at the heart of a number of fields today, like computer science, artificial intelligence, and of course neurology. It's not that they are all the same, but understanding one might teach us something about the others.

"I don't think that ant colonies work like brains, or that embryos work like ant colonies, but I do think the more we understand how any system like this works, the more we might be able to ask the right questions about another one," she adds.

And that took her to the big, dark harvester ants of the Southwest. She picked that species — one of about 10,000 in the ant world — because they are easy to find, and easy to see.

Her first years of research taught her lots, like how the colonies are organized to carry out certain chores. The youngest ants take care of the nest. Later, they graduate to the patroller class, going forth to find sustenance for the colony. And finally they become foragers, a high risk assignment given to the senior members, possibly because they're reached an advanced age and aren't going to be around much longer anyway.

The colony also includes a queen, whose sole function is to produce thousands of other ants every year. Although we call her the queen, she has no authority over the colony.

So how does work get done? How can you have a war with no generals?

Gordon's earlier research had suggested that the ants communicate by smell, but Green — the lead author in the Nature report — wanted to take it a step farther.

Gas Signals

Ants from all three classes — nesters, patrollers and foragers — were examined with a sophisticated piece of equipment. Gas chromatography revealed that each ant was coated with about 25 different hydrocarbons which emit slightly different odors.

"Hydrocarbons are simply molecules of hydrogen and carbon," Greene says. "There's nothing fancy about them. Yet subtle changes in the concentration of these relatively simple chemicals can produce very important and profound behavioral changes in ants."

It turned out that a patroller emits a slightly different gas than a forager, thus giving each a distinctive odor, at least to another ant. But is that really what sets the work day in motion? To find out, the researchers resorted to a clever bit of deception.

Tiny glass beads were coated with hydrocarbon that would give them the distinct odor of a patroller. Then the researchers hit nine different harvester ant colonies in Arizona, collecting the patroller ants early in the morning before they had a chance to return to the nest. Some 30 minutes later they dropped the tiny beads into the nest, one at a time, about 10 seconds apart.

The foragers brushed the beads with their antennae, and then took off for their day's work.

It didn't work when beads that smelled like foragers or nesters were dropped into the nest. It had to be patrollers, and the beads had to arrive at precisely the right frequency.

That proves, Gordon says, that the right smell, delivered in the right dosage, is all it takes to keep order in the system. The patrollers took off to do their thing without anyone having to tell them to do so.

It's not known yet whether other ant species use the same mechanism, she adds. Of the thousands of ant species all over the world, only about 50 have been studied extensively, Gordon estimates.

Who knows what dark secrets some of those may harbor? Perhaps some of them might help us understand how to build a robot that could make its own repairs, and reach its own decisions, and carry out tasks millions of miles away from home without someone pushing a single button.

Lee Dye’s column appears weekly on A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.